The use of concrete-filled steel tubular (CFST) columns has increased in recent decades due to their excellent structural performance which takes advantage of the combined effect of steel and concrete working together. The steel tube provides confinement to the concrete core resulting in increased compressive strength whereas the concrete core restricts inward deformation of the steel tube thus enhancing local buckling resistance of steel tubes. In addition to these advantages, the steel tubes surrounding the concrete columns eliminates permanent formwork which reduces the construction time and costs. Moreover, CFST columns possess better fire resistance as compared to steel columns
The fire risk in high-rise buildings is significantly higher than a lower-rise building because of the potential for more fire locations and greater consequences of the fire itself (e.g. stack effect) and to a greater number of occupants. However, the fires in high-rise buildings generate large quantities of smoke that can spread vertically or horizontally through the building even if the fire is contained to only one room or unit. The smoke emitting from burning polypropylene fibers usually embedded in high strength concrete adds more toxicity to the smoke which is responsible for many of the fatalities in such incidents. Thus, there is higher potential risk to life from fire in high-rise buildings which demands greater fire safety in these buildings where CFST columns are commonly used for carrying heavy loads.
There are a number of approaches for the construction of such columns to overcome the problem with such construction as for example in the escape of smoke and gases during exposure to fire especially when the columns are massive. The problem gets aggravated with the use of high-strength concrete because of the reduced porosity thus providing fewer escape routes for gases during fire exposure. The mixing of polypropylene fibers in the high-strength concrete of reinforced concrete members helps to provide passages by the melting of fibers during fire for the escape of gases from inside a concrete mass. But, the mixing of polypropylene fibers in concrete of CFST columns will not be that effective because of the requirement of a large number of vents required in the steel tube which is not structurally favorable. The exposure of such columns to fire may lead to more serious consequences in the case of insufficient vents. Therefore, most of the available studies use a plurality of vents in the outer steel column for the escape of gases.
A U.S. Pat. No. 8,484,915 of Abbas et al. entitled “System for Improving Fire Endurance of Concrete-Filled Steel Tubular Columns” is assigned to the same assignee as the present invention. As disclosed therein, a concrete filled tubular steel column includes a longitudinally extending outer vertical tubular steel shell and an inner perforated tubular steel shell disposed at the center of the outer steel shell to be coaxial therewith. A plurality of spaced vertically steel plates extend from the inner steel shell towards but not abutting the outer steel shell. In addition, a plurality of horizontally disposed perforated pipes extend outwardly from the inner member. All perforated inner tubular steel shell and pipes have a plurality of meltable polymer plugs or caps to prevent plastic cement from flowing into or closing the openings. In the event of fire, the plastic or polymer plugs or caps melt and allow gases and smoke to flow into the perforated pipes and up through the inner member and out therefrom to the top of the column
It is presently believed that there is a present need and a potential commercial market for an improved concrete-filled steel tubular column for high load carrying capacity and fire resistance. There should be a need and a potential commercial market for such products that dissipate smoke and toxic gases at the top of the column, eliminate to a large degree exit vents in the steel tubular column and to a larger degree reduce the smoke and toxic gases from areas adjacent to inhabited floors as well as reducing the costs while maintaining the load carrying capacity.